The invention relates to the inhibition of corrosion of iron and of ferrous metals in carbonic brines. For the purposes of the invention, the term carbonic brines is understood to refer to carbon dioxide-charged aqueous solutions of inorganic salts.
(C10-C22)Alkyl-poly(ethyleneamino)imidazolines (or 2-alkyl poly-3-(ethyleneamino)-1,3-diazolines) are known to be excellent corrosion inhibitors for iron and ferrous metals in sulphydric brines, or sulphydric and carbonic brines, and many publications have been devoted to the mechanism of this inhibition. To mention but the most recent: Preparation of Corrosion Inhibitor for Water Flooding in Oilfield and Study of its Mechanism, Lu Zhu et al., Water Treatments, 8 (1993), 253-264, China Ocean Press or Hydrogen Sulphide Corrosion of Steel, Mechanism of Action of Imidazoline Inhibitors, A. J. Szyprowski, Proceedings of the 8th European Symposium on Corrosion Inhibitors, Ann. Univ. Ferrara, N. S., Sez. V, suppl. N.10, 1995. However, curiously, these compounds are very ineffective in corrosive media containing only CO2, which are nevertheless apparently less aggressive, and this motivated the search for more complex similar structures such as N,Nxe2x80x2-substituted imidazolines (U.S. Pat. No. 5,322,640) or adducts of imidazolinones and of urea (GB2,190,670).
It has now been found that salts of mercapto acids and of alkyl-polyethyleneamino-imidazolines are very powerful corrosion inhibitors for iron and ferrous metals in carbon dioxide-charged aqueous solutions of inorganic salts.
The present invention consists of corrosion-inhibiting compositions containing at least one alkyl-poly(ethyleneamino)-imidazoline or 2-alkyl-poly-3-(ethyleneamino)-1,3-diazoline, corresponding to the general formula 
in which R is a linear or branched, saturated or unsaturated hydrocarbon chain containing 10 to 22 carbon atoms, and in which n is a number from 0 to 3, and at least one mercapto acid corresponding to the general formula 
with
n=0 to 3,
R1=H or SH,
R2 and R3, together or independently, =C1-4, CON(R6)(R7) or COOR8,
R4 and R5, together or independently, =OH, NH2 or SH when R1xe2x89xa0SH,
R6 and R7, together or independently, =H or C1-4,
R8=H or C1-8,
it being possible for R2 to R5 to be included in an aliphatic ring,
it being possible for R3 and R5 to be included in an aromatic ring when n=1,
A being a COOH, SO3H, OSO3H, PO3H or OPO3H acid group,
the molar ratio between the mercapto acid component(s) and the imidazoline component(s) being from 1.0 to 1.5.
The alkylimidazolines forming part of the composition of the inhibitors of the invention are products of condensation-cyclization of saturated or unsaturated fatty acids containing 10-22 carbon atoms, with polyethylene-polyamines (diethylenetriamine DETA and its higher homologues, triethylenetetramine TETA, tetraethylenepentamine TEPA and pentaethylenehexamine PEHA). These imidazolines are obtained under conditions that are well known to those skilled in the art (introduction of the polyamine into the molten fatty acid, in the presence of oxalic acid which acts as cyclization accelerator, raising the temperature to 200xc2x0 C.-220xc2x0 C. and maintaining it at this stage until the water of condensation has been entirely removed).
The preferred mercapto acids are mercaptocarboxylic acids,
HSxe2x80x94(CH2)nxe2x80x94COOH
and among these mercaptoacetic acid,
HSxe2x80x94CH2xe2x80x94COOH
and mercaptopropionic acid
HSxe2x80x94CH2xe2x80x94CH2xe2x80x94COOH.
The compositions according to the invention are advantageously in the form of aqueous solutions containing 10%-75% active material. Solutions containing 20-40% by weight in water or in a mixture of water and a water-miscible solvent are preferred, isobutanol, butyl glycol, monoethylene glycol or mixtures thereof being preferred solvents. They are prepared very simply by mixing the constituents together while cold or warm. They are used at effective concentrations of 5 to 200 ppm, preferably at about 20 ppm of the preparation (calculated as imidazoline mercaptocarboxylate component relative to the corrosive medium).
These water-soluble compositions, modelled on the traditional but insufficiently effective imidazoline acetates, inhibit the static corrosion of iron and ferrous metals in carbonic brines of high salinity at high temperature. They also inhibit its dynamic corrosion under the conditions for the production of gas, and their efficacy is maintained in two-phase media such as carbonic brines/oils. However, in order to enhance their performance when the hydrocarbon phase is a crude petroleum oil and when the aqueous phase is highly saline, i.e. when its salt concentration is greater than 1 g/l, they are advantageously combined with cationic, amphoteric and nonionic surfactants from the group consisting of alkyltrimethyl- or alkyldimethylbenzylammonium salts, alkylaminopropionic acids and oxyethylenated alkylamines, in which the alkyl chain is a saturated or unsaturated chain containing from 12 to 22 carbon atoms. This effect is unexpected; it is not impossible for it to be due to the improvement to the partition of the mercaptocarboxylate/imidazoline system between the oily phase and the aqueous phase.
The mercaptocarboxylate/imidazoline/surfactant compositions typically comprising 20 to 40% imidazoline mercaptocarboxylate, 2 to 5% quaternary ammonium, 2 to 5% amphoteric surfactant and 2 to 10% oxyethylenated amine are also subjects of the present invention.